(1) Field of the Invention
The present invention relates to a rotary anode for X-ray tubes, and to a method for producing it.
(2) Description of the Related Art
As the rotary anode, hereinafter referred to as target, for X-ray tubes, heretofore use is made of a two-layered structure composed of an X-ray generating layer of a high-melting-point metal of pure tungsten (hereinafter referred to as pure W) or a rhenium-tungsten (hereinafter referred to as Rexe2x80x94W) alloy and an underlying substrate of pure molybdenum (hereinafter referred to as pure Mo) or TZM (this indicates an alloy of 0.5% Ti-0.07% Zr-0.05% C-balance of Mo) as laminated together.
For producing the conventional target, pure W powder or a mixed powder of Re powder and W powder, which is previously mixed with an organic binder to form a material powder. The material powder is put into a mold, and lightly compressed therein from the upper and lower sides. Thereafter, the material powder is stacked with an additional material powder which consists of a predetermined amount of Mo powder or a mixed powder to give a composition of TZM. The additional material powder is previously mixed with an organic substance and, thereafter, put into the mold to form a stacked material body. The stacked material body is compressed therein from the upper and lower sides to give a two-layered disc molding.
Next, the organic substance is removed from it in a hydrogen atmosphere at a temperature falling between 300 and 500xc2x0 C. Thereafter, the molding is sintered in hydrogen at 1800xc2x0 C. to form a sintered body. The density of the sintered body generally falls between 90 and 95%.
For increasing its density and for making it have an umbrella-like shape, the sintered body is then subjected to plastic working of, for example, hot rolling and/or hot forging to thereby make it have an umbrella-like shape nearly approaching its final shape, and thereafter this is machined to have a final target shape. In the last step, the thus-shaped target is degassed in vacuum at a temperature of around 1500xc2x0 C. for the purpose of removing the gaseous component from it. After those steps, the intended target is produced.
However, targets are used under severe conditions, for example, at high temperatures and at high rotating speeds, e.g., at 10,000 rpm. Therefore, the targets are desired to be high quality. In particular, as they shall generate X rays in high vacuum, their life is greatly shortened if the vacuum degree around them is lowered. In addition, if the organic binder used could not be completely removed from them during their production, it remains in them as a carbon residue. In that condition, the targets themselves are heated at high temperatures owing to thermions dashing thereon, and will be dead in a lowered vacuum degree.
Moreover, the conventional process requires long and complicated steps, and also requires expensive raw materials of W, Re and Mo in a large amount of from 3.0 to 4.0 times the weight of the final products. The process thus requiring such a large amount of natural resources and even much energy could not be one that is gentle to the environment. Furthermore, the process could not meet the current requirements, as being uneconomical and expensive.
It is an object of the present invention to provide a method for producing a high-quality and high-reliability rotary anode for X-ray tubes, which does not require any vapor-generating component in its production steps.
It is another object of this invention to provide a rotary anode for X-ray tubes, which is inexpensive and gentle to the environment.
It is still another object of this invention to provide a method for producing the rotary anode for X-ray tubes.
Before completion of this invention, we, the present inventors have developed a method for producing a rotary anode target, hereinafter referred to as a simple term of xe2x80x9ctargetxe2x80x9d, for X-ray tubes, in which the grain size of the Rexe2x80x94W layer and that of the Mo powder for the target are optimized, the contraction of the target being sintered after isostatic powder molding is unified, and the carbon residue in the target is reduced through isostatic molding not requiring any organic substance, and have found an economical method for producing a high-quality and long-life target that is well applicable to high-speed rotation use in high-temperature and high-vacuum conditions. The method requires reduced amounts of raw materials and shortened and simplified steps. On the basis of these findings, we have completed the present invention.
According to one aspect of the present invention, there is provided a rotary anode for X-ray tubes having a two-layered structure composed of a Mo-containing layer and a Wxe2x80x94Re alloy layer laminated to the Mo-containing layer. The Mo-containing layer consists essentially of Mo or an Mo alloy.
In this aspect of the present invention, preferably, the Mo-containing layer is a substrate, and the Wxe2x80x94Re alloy layer is an X-ray generating layer that overlies the substrate. Also preferably, the Mo-containing layer is comprised of at least one of TiC, HfC and ZrC in an amount of from 0.2% by weight to 1.5% by weight, and the balance of substantially Mo.
In this aspect of the present invention, still preferably, the substrate has a bending strength at 700xc2x0 C. of 800 MPa or more, and has a tensile strength at 1000xc2x0 C. of 300 MPa or more.
According to another aspect of the present invention, there is provided a method of producing a rotary anode for x-ray tubes. The rotary anode has a two-layered structure composed of a Mo-containing layer and a W-Re alloy layer laminated to said Mo-containing layer. The Mo-containing layer consists essentially of Mo or an Mo alloy.
In the aspect of the present invention, the method comprises a step of filling a W-containing powder and a Mo-containing powder into a mold to give a two-layered structure, the W-containing powder consisting essentially of W powder and Re powder, the Mo-containing powder consisting essentially of Mo powder or Mo powder and at least one of TiC powder, HfC powder and ZrC powder, followed by isostatically molding it to prepare a compacted body nearly approaching its final shape, a first sintering step of sintering the compacted body in a hydrogen atmosphere into a first sintered body, a second sintering step of further sintering the first sintered body in vacuum into a second sintered body, and a machining step of machining the second sintered body into the intended rotary anode.
In the aspect of the present invention, the method comprises a step of filling a W-containing powder that comprises W powder and Re powder, and an Mo-containing powder that comprises Mo powder, or comprises Mo powder and at least one of TiC powder, HfC powder and ZrC powder, into a mold to give a two-layered structure, followed by isostatically molding it to prepare a compacted body nearly approaching its final shape, a first sintering step of sintering the compacted body in a hydrogen atmosphere into a first sintered body, a second sintering step of further sintering the first sintered body in vacuum into a second sintered body, and a machining step of machining the second sintered body into the intended rotary anode.
In this aspect of the present invention, it is preferable that the starting materials of W powder, Re powder and Mo powder have a mean grain size falling between 1 and 5 xcexcm.
In the invention, TiC is thermally stable and enhances the intergranular strength of Mo, thereby improving the strength of the Mo-containing structure at room temperature and even at high temperatures. In addition, even when the TiC-containing material is exposed to high temperatures, the grains constituting it are prevented from growing into coarse and large grains. For these reasons, therefore, it is preferable that the Mo-containing layer and its material of Mo-containing powder contain TiC.
However, when the TiC content of the Mo-containing powder is smaller than 0.2% by weight, the effect of TiC therein to enhance the intergranular strength of Mo will be poor, and, in addition, TiC could hardly prevent the grains from growing into coarse and large ones at high temperatures. The Mo-containing powder having such a small TiC content will be substantially the same as pure Mo. On the other hand, when the TiC content of the Mo-containing powder is larger than 1.5% by weight, the relative density of Mo in the substrate will be low, thereby often resulting in that the substrate is cracked especially during plastic working to lower the yield of the product.
For these reasons, therefore, it is desirable that the Mo-containing powder is prepared by adding TiC powder to Mo powder in an amount of from 0.2% by weight to 1.5% by weight.
In the invention, also preferably, the Mo alloy substrate layer and the X-ray generating layer of a Wxe2x80x94Re alloy are formed according to a process that comprises press-molding the raw materials for the two layers through powdery metallurgy into a two-layered green compact, then subjecting the green compact into first sintering in a reducing atmosphere of, for example, hydrogen or the like at a temperature falling between 1500 and 2100xc2x0 C. continuously followed by second sintering in an inert atmosphere or in vacuum at a temperature higher than the first sintering temperature, and thereafter degassing the thus-sintered body at a temperature lower than the second sintering temperature.
In the invention, it is further preferable that the sintering step in the second sintering step falls between 1800 and 2200xc2x0 C. and that the vacuum degree in the vacuum sintering falls between 10xe2x88x926 Torr and 10 Torr.